The invention relates to a heat exchanger, especially for motor vehicles, which consists of a plurality of fins disposed in a parallel, spaced relationship, having openings and collars of oval or planar cross section adjoining the openings, and of a plurality of tubes of corresponding cross section disposed perpendicularly thereto and passing through the openings and collars. The tubes are joined to the fins by expanding their cross section after they have been inserted into the openings and collars thereby forcing their walls against the margins of the openings and collars.
Heat exchangers of this kind are a particular kind of tube radiator. They differ from conventional tube coolers in that the tubes are joined to the fins only by expanding their cross section and therefore they are not additionally soldered, welded or cemented to the fins. To be effective it is necessary that the tube walls be fully engaged with the edges of the openings or with the collars along their entire periphery, because otherwise the heat conduction is too greatly impaired. This full engagement is achieved when the tubes are expanded, by upsetting the collars of the fins slightly, and in the elastic and partially plastic range.
In the use of tubes of circular cross section, this type of joining is problem-free. But on account of the lack of streamlining it is not advantageous to use round tubes. Tubes of oval or flat-oval cross sections have the advantage of a streamlined shape. However, it has been possible only to a limited extent to assemble tubes of this kind by expansion. It is known, in the case of tubes of oval cross section, that the expanding technique can be used as long as the ratio of the longest diameter to the shortest diameter is not greater than about 4:1. Above this limit, however, the quality of the junctions produced by expansion decreases greatly, especially because the tubes collapse along their broad sides, even if they are expanded more strongly than in the area of their narrow sides. Aside from this, as in the case of round tubes, only tubes of relatively small longest diameters can be used, because otherwise the tubes would have to exceed considerably the wall thickness of about 0.4 mm, which, for reasons of cost, is about the maximum acceptable thickness. As a result, tubes of oval cross section, if they are to be joined to the fins by expansion, have heretofore been arranged basically in several planes.
Lastly, in the case of flat-oval tubes having two planar, parallelly disposed broad sides joined at their ends by semicylindrical sections, joining by expansion will not work at all.
The problem to which the invention is addressed is to create a heat exchanger of the kind identified above in which tubes can be used which, even with small wall thicknesses, have relatively great longest diameters and relatively small shortest diameters. In particular, a method for the manufacture of such heat exchangers is to be given whereby the tubes and fins of the heat exchanger can be joined together in a simple manner and good contact is achieved, especially between the walls of the collars and the walls of the tubes. Lastly, an apparatus is also to be proposed, which will be suitable for the series production of the heat exchangers according to the invention.